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A flooded section of tunnel in Auckland’s Central Rail Link project. (Photo: Supplied)
A flooded section of tunnel in Auckland’s Central Rail Link project. (Photo: Supplied)

ScienceFebruary 3, 2023

How do we know climate change played a role in the Auckland floods?

A flooded section of tunnel in Auckland’s Central Rail Link project. (Photo: Supplied)
A flooded section of tunnel in Auckland’s Central Rail Link project. (Photo: Supplied)

The science of ‘event attribution’ is growing, with researchers working to accelerate their assessments. A leading NZ climate scientist tells Toby Manhire how it works, how climate change impacted the ‘off the chart’ weekend downpours, and why we can’t put a number on it tomorrow. 

Brutal, unexpected, record-breaking, destructive, tragic. The torrential rainfall that engulfed Auckland on Friday night is all those things, and, as is now widely accepted, linked to climate change.

“It’s a one-in-100-year weather event, and we seem to be getting a lot of them at the moment,” said PM Chris Hipkins. “I think people can see that there’s a message in that … Climate change is real, it’s with us.” After initially expressing uncertainty, mayor Wayne Brown jumped on board. “As the prime minister has said, this is climate change,” he stated. “And I agree.”

Similarly, media reports increasingly note the climate change link. Analysis by Dot Loves Data for The Spinoff found a lift in references to climate change in Auckland related coverage of 255% across the month of January, with 42 mainstream articles making mention of climate change in flooding coverage, “showing that the nexus between the two is strong”. 

But how did climate change play a role? To what extent? And how do scientists go about working that all out? 

The gargantuan event

“We use a variety of tools,” says Sam Dean, the principal climate scientist at Niwa. “We can look at all the observations on what has fallen over the last 100 years. That helps us to understand the rainfall climate of a place.” It only helps up to a point, however. The Auckland deluge on Friday – “what fell in under six hours is just gargantuan for sea level in New Zealand” – was not just exceptional. It was “off the charts”, says Dean. 

Happily, scientists can turn to another approach: wrapping up the different component parts of an extreme weather event and chucking them into a computer to see what happens in, by turn, a world without warming, the heated world in which we live, and a world that has heated further.

Sam Dean, principle climate scientist at Niwa.

“Just like weather forecast models, which are these complicated models that incorporate all our knowledge of how weather and the atmosphere works, and the ocean and the land and how they all interact, we run those models for much longer periods of time,” Dean explains. “We use that to do climate predictions. We run them out for the next 100 years. We put into them the scenarios of greenhouse gas going up and look at how that changes the world. But we can also do a counterfactual experiment, in which we create a hypothetical world: the world today if there had never been any emission of greenhouse gasses, but everything else is the same.”

The models are then run thousands of times on supercomputers, generating “lots of versions of the world, providing a really good understanding of the likelihood of extreme events occurring.”

At this point, I’d like to be able to tell you that Dean, looking up from the supercomputer, told us just how much climate change increased the likelihood and intensity of the Auckland dump. He can’t, of course – there is a huge amount of time and research entailed by such an undertaking, starting with sizing up the nature of Friday’s extreme event. “Those kind of experiments take a while to do, and they are very specific to the location,” he says.

Researchers have judged that climate change exacerbated floods including those in Canterbury and Westport 2021, for example, but the studies took many months to complete. A paper published last year that examined five extreme weather events in New Zealand and the use of different models to assess what scientists call “extreme event attribution”, assessing the role human-induced cause climate change played, noted the “growing interest in making rapid assessments of extreme events within operational forecast centres”.

The push for faster assessments of the role climate change is likely to have played in specific bouts of extreme and harmful weather events is a global one. At the forefront is the World Weather Attribution initiative, an international group of scientists focused on issuing attribution assessments and doing so expeditiously, while the event itself is still fresh in public minds. Friederike Otto, one of the project’s leaders from Imperial College in London, told the January edition of Wired magazine: “I see attribution now as a tool that helps us disentangle drivers of disasters and helps us use extreme events as a lens in society to see where we are vulnerable.”

WWA scientists assess two central factors: how much climate change made an event more likely, and how much climate change intensified its force. A study of extreme rainfall that hit vulnerable communities in West Africa between May and October last year, for example, was published by WWA in mid-November. Its modelling of severe rainfall over the lower Niger Basin found that “climate change made the event about twice as likely and approximately 5% more intense”, while the conclusion on downpours around Lake Chad was that “climate change made the event about 80 times more likely and approximately 20% more intense”.

Mt Roskill on Friday night. (Photo: The Spinoff)

Convection centre

What kind of an extreme event was the Auckland downpour? Convection plays a big part, says Dean. “The media people keep telling us nobody likes the word convection, because they don’t know what it means,” says Dean with a gallows laugh. Give it a shot. “It’s when you get these kind of cells in which the heat is generating lots of lift, releasing heat.” The clash between that heat and its surrounding creates a cycle of powerful energy, “releasing heat from the condensing of the water, and the conversion of the water from gas to liquid is releasing more heat, and that’s driving more lift. It’s like a turbocharged runaway process that generates really, really heavy rain. You get that a lot in the tropics. You don’t get it here very much, just a bit in the north of the country occasionally.”

He says: “Has climate change made it possible for such events to occur in New Zealand with some regularity? I don’t know the answer to that … But convective storms, generated from that extra heat in the atmosphere and that extra moisture do increase as a result of climate change.”

Chuck into the mix high pressures sitting over the country, a “blocking front” hanging around to the east, the La Niña effect more widely and a marine heatwave, and, well, “extreme events happen in extreme situations”. A “convective train”, he says, “ran down the lines through Auckland over and over again during those six hours”.

If an event such as Friday’s downpour were a vicious soup, climate change could be imagined as a fiery element beneath it – but also, perhaps, one of the binding agents. “Climate change supercharges the whole thing. But it could also be changing the likelihood of those ingredients coming together. We wouldn’t know the answer to that without a detailed study.”

Dean is chomping at the bit to launch a study on what happened in Auckland, on what it all means. The critical question, he says, is this: “Whether there is a permanent change in the climate of Auckland that will lead to much more frequent occurrences of such events. This is exceptional. It’s off the charts in terms of short duration rainfall for Auckland. Can we go another 200 years without it happening again? Or is it something that’s now going to happen more regularly?”

The shock felt by those in Auckland and other affected areas in the North Island was reflected among the scientists. “I know some of our observational guys are kind of still sitting there with their mouths open, because how do you describe such an amount of rainfall?” says Dean. “And what does it mean? And when will it happen again? And I’m really sorry that we don’t have all the answers to those questions.” He takes a breath. “It has in some sense caught all of us by surprise. But, you know, climate change is putting more energy and more moisture into the atmosphere. That is definitely making these things more intense.”

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